The present invention relates to a liquid crystal display material, a liquid crystal display method, and a liquid crystal display device. More particularly, the present invention relates to a liquid crystal display material comprising a polymer in which a liquid crystal is dispersed in dissolvable state or phase-separable state, a liquid display method utilizing thermal phase separation in which the image forming state can be maintained by keeping the liquid crystal in translucent state by controlling the cooling rate, and a liquid crystal device using the liquid crystal display material.
Development of liquid crystal display devices using polymer dispersion type liquid crystals has been in progress. The polymer dispersion type liquid crystals are superior in properties such as luminosity, contrast and angle of visual field to liquid crystal display devices utilizing polarized light. Furthermore, due to the structure of the liquid crystal being dispersed in a polymer, the liquid crystal is not needed to be sealed and it is easy to make larger the area of the devices.
Such conventional polymer dispersion type liquid crystal displays utilize changing of optical properties of the liquid crystals due to response to electric field or response to heat. For example, JP-A-3-52843 discloses a liquid crystal optical device using the polymer dispersion type liquid crystal with utilization of electric field response. In this device, ordinary light refractive index of the liquid crystal and that of the polymer are close to each other, and thus an electric field is applied to the liquid crystal to arrange the orientation of liquid crystal molecules in the direction of application of the voltage, thereby to make the device transparent, and during application of no electric field, the liquid crystal molecules align along the interface of polymer/liquid crystal, and, hence, light is scattered at the interface due to the difference in refractive index of the polymer and the liquid crystal, thereby to result in cloudy state.
However, in the case of this device, if thickness of the polymer dispersion type liquid crystal layer is increased so as to increase cloudiness with no electric field being applied, the driving voltage simultaneously rises. Moreover, even during an electric field being applied, since fine liquid crystal droplets cannot respond to the electric field, or since it is difficult to make the ordinary light refractive index of the liquid crystal close to that of the polymer, a complete transparent state can hardly be obtained. That is, for attaining a display having excellent contrast with low voltage driving, there is an electro-optical limit.
Furthermore, Display Material Investigative Report II (edited by a corporation Japan Electronic Industry Promotion Association and published in March, 1991; Items 85-97) discloses a liquid crystal device utilizing a heat responsive polymer dispersion type liquid crystal. This liquid crystal device uses a polymer dispersion type liquid crystal comprising a polymer in which a nematic liquid crystal is dispersed. FIG. 1A shows an unheated state (nematic phase) of the above polymer dispersion type liquid crystal, and FIG. 1B shows a change in mode of the polymer dispersion type liquid crystal in heated state (isotropic phase). FIG. 2 shows the relation between temperature and refractive index of the polymer dispersion type liquid crystal.
As shown in FIG. 1A and FIG. 2, the polymer dispersion type liquid crystal is in cloudy or opaque state in unheated state. This is because light is scattered at the interface of polymer/liquid crystal as in the above-mentioned liquid crystal display devices using the electric field driving polymer dispersion type liquid crystals.
On the other hand, as shown in FIG. 1B and FIG. 2, when this polymer dispersion type liquid crystal is heated and just after the temperature exceeds a certain point (nematic-isotropic phase transition point: TNI), the polymer dispersion type liquid crystal changes from cloudy state to transparent state. This is because the liquid crystal in the polymer dispersion type liquid crystal is heated to higher than the nematic-isotropic phase transition point to lose liquid crystallinity and to cause decrease of difference between the refractive index of the isotropic phase and the refractive index np of the polymer. In this case, heat conduction uniformly occurs without depending on the particle diameter of the liquid crystal droplets and, hence, display driving property is satisfactory, but since it is also difficult to completely make the refractive index of the isotropic phase close to that of the polymer, contrast is not sufficient.
Moreover, xe2x80x9cLiquid Crystal Display Techniquexe2x80x9d (edited by Industrial Investigation Association and published on Sep. 20, 1994; Items 53-57) and others report thermal writing devices using both the electric field and the heat. For example, JP-A-6-18831 discloses a display device fabricated by sandwiching a layer comprising a nematic liquid crystal dispersed in a polymer between a transparent electrode and a substrate provided with a heating wiring. In this device, orientation of the liquid crystal in the direction of electric field is fixed to maintain the transparent state by using heat and electric field in combination. In this case, too, for performing a display excellent in contrast by low voltage driving, there is an electro-optical limit, and, besides, the transparent state cannot keep a high transmission over a long period of time.
Thus, an object of the present invention is to provide a novel liquid crystal display material having an excellent contrast ratio.
Another object of the present invention is to provide a polymer dispersion type liquid crystal display method and device which are capable of thermal writing/thermal erasing by maintaining a translucent state.
The above first object can be attained by a liquid crystal display material comprising a polymer in which a liquid crystal is dispersed in dissolvable or phase separable state, wherein the liquid crystal dissolves in the polymer at high temperatures and separates from the polymer at low temperatures, and a contrast ratio (B/A) of a reflectance (A) of the display material cooled from the dissolution state at a rate of 10xc2x0 C./sec and a reflectance (B) of the display material cooled from the dissolution state at a rate of 4xc2x0 C./sec is not less than 2.
The above second object of the present invention is attained by a polymer dispersion type liquid crystal display where a display medium having a material composition according to which the liquid crystal disperses in the polymer in unheated state and the liquid crystal dissolves in the polymer in heated state is used and the cooling rate from the dissolution state is adjusted to control the particle diameter of liquid crystal droplets, thereby maintaining the image forming state.